I can understand the Doctor not really understanding about human racism (except in a 'yet another example of why they're unevolved and stupid' sort of way), which could have been AWESOME. He takes Martha back to racistville and Martha could have been all 'Er, Doctor? This is not going to work well for me, you know.' And it could have been thinking and great and a bit History of Racism for Beginners. But no, the show decided not to do that, for some reason.